
SCIENCE172
Fig. 13.15 : Demonstration of blind spot
Caution : For this activity, never use
a laser torch.
Activity 13.8
Look into your friend’s eye. Observe
the size of the pupil. Throw light on
her eye with a torch. Observe the
pupil now. Switch off the torch, and
observe her pupil once again. Do
you notice any change in the size
of the pupil? In which case was the
pupil larger? Why do you think it
was so?
In which case do you need to
allow more light in the eye, when
the light is dim or bright?
Behind the pupil of the eye is a lens
which is thicker in the centre. What
kind of lens is thicker at the centre?
Recall what you learnt about lenses in
cornea (Fig. 13.14). Behind the cornea,
we find a dark muscular structure
called iris. In the iris, there is a small
opening called the pupil. The size of
the pupil is controlled by the iris. The
iris is that part of eye which gives it its
distinctive colour. When we say that a
person has green eyes, we refer actually
to the colour of the iris. The iris controls
the amount of light entering into the
eye. Let us see how.
Iris
Lens
Ciliary
muscle
Cornea
Retina
Optic
Nerve
Fig. 13.14 : Human eye
Class VII. The lens focuses light on the
back of the eye, on a layer called retina
(Fig. 13.14). The retina contains several
nerve cells. Sensations felt by the nerve
cells are then transmitted to the brain
through the optic nerve. There are two
kinds of cells–
(i) cones, which are sensitive to bright
light and
(ii) rods, which are sensitive to dim light.
Cones sense colour. At the junction
of the optic nerve and the retina, there
are no sensory cells, so no vision is
possible at that spot. This is called the
blind spot. Its existence can be
demonstrated as follows:
Activity 13.9
Make a round mark and a cross on
a sheet of paper with the spot to the
right of the cross (Fig. 13.15). The
distance between two marks may be
6-8 cm. Hold the sheet of paper at
an arm’s length from the eye. Close
your left eye. Look continuously at
the cross. Move the sheet slowly
towards you, keeping your eye on
the cross. What do you find? Does
the round mark disappear at some
point? Now close your right eye.
Look at the round mark now and
repeat the activity. Does the cross
disappear? The disappearance of the
cross or the round mark shows that
there is a point on the retina which
cannot send messages to the brain
when light falls on it.